16318
J. Phys. Chem. 1996, 100, 16318-16326
Support and Temperature Effects in Platinum Clusters. 2. Electronic Properties Boyan I. Boyanov*,† and Timothy I. Morrison Department of Physics, Illinois Institute of Technology, Chicago, Illinois 60616 ReceiVed: May 14, 1996; In Final Form: June 26, 1996X
The effects of Brønsted acidity on the spatial structure and electronic properties of platinum clusters supported on zeolite Y have been examined with X-ray absorption (XANES and EXAFS) and X-ray photoelectron spectroscopy. The clusters contain 10-25 Pt atoms on average, with a nearest-neighbor distance of 2.70 ( 0.01 Å. Static disorder in the atomic distributionswhile certainly present in the supported metalsis shown to be symmetric on average. Increasing Brønsted acidity of the zeolite support has no measurable systematic effect on the spatial structure of the clusters but results in reproducible 5-10% enhancement of near-edge features in the L2,3 X-ray absorption spectra, as well as 0.2-0.3 eV shifts in Pt 4f and 4d core-level binding energies and valence-band thresholds. Evidence is presented that the interaction between the electronic levels of the cluster and the support is not dominated by charge-transfer effects but results in the creation of unoccupied antibonding states above the Fermi level. Structure of atomic origin isolated from the EXAFS data exhibits no dependence on the measurement temperature and the acidity of the zeolite support. The implications of these results for current XANES-based methods for estimation of valence-band charge count are discussed.
I. Introduction The first part of this paper1 described the primary goals and motivation for this work and presented our analysis of the spatial structure of zeolite-supported metal clusters. The clusters were found to contain fewer than 25 Pt atoms, with nearest-neighbor Pt-Pt distances of 2.70 ( 0.01 Å, in agreement with previous work.2-5 It was shown that static disorder, while certainly present in supported metals, is symmetric on average. The measured bond contractions (0.07 Å) are therefore deemed to be intrinsic to the cluster. Within the resolution of this work, zeolite acidity was found to have no systematic effect on the spatial structure of the supported metal. In addition to providing information on the spatial structure of the supported metal, the near-edge region of the X-ray absorption measurements described in part 1 of this paper1 may be used to monitor small modifications in the electronic structure of the metal, e.g., charge-transfer and hybridization effects. It is our goal (a) to determine whether increasing zeolite acidity results in systematic trends in the electronic structure of the supported metal and (b) to determine the origin of the observed trends. II. Experiment and Data Reduction A. X-ray Absorption. The sample nomenclature, preparation, and characterization procedures, as well as the experimental conditions during the X-ray absorption measurements, have been described in part 1 of this paper1 and will not be repeated here. The dependence of the intensity of the L2,3-edge white lines on the amount of d-band character is well-documented,7-10 and XANES difference spectra are often used to characterize small modifications in the chemical state of the absorber atom.11,12 However, XANES difference spectra are known to be extremely sensitive to the harmonic content of the primary X-ray beam,6 * To whom correspondence should be addressed. † Present address: Department of Physics, North Carolina State University, Box 7518, Raleigh, NC 27695. X Abstract published in AdVance ACS Abstracts, September 15, 1996.
S0022-3654(96)01401-3 CCC: $12.00
sample thickness,13 spectrum alignment,13 and even the measurement temperature.12 The procedure used in this work to calculate XANES difference spectra closely follows previously published prescriptions.13 If the radial integral R2(E) that enters in the calculation of the X-ray absorption coefficient14 is assumed to depend only weakly on the photoelectron kinetic energy E, the normalized XANES difference spectra of chemically similar compounds should be representative of the absolute changes in the unoccupied DOS above the Fermi level. On the basis of theoretical treatment of the band structure of 5d transition metals by Mattheiss and Dietz,7 the following expressions may be written for the integrated intensities A2 and A3 of the L2- and L3-edge bound-state resonances:7,11 1/2 2 A2 ) ∫µ2(E) dE ) CNa(R2p d ) h3/2/3
(1)
3/2 2 A3 ) ∫µ3(E) dE ) CNa(R2p d ) (h3/2 + 6h5/2)/15
(2)
Here C is a universal constant, Na is the number density of the absorber, Rid are radial integrals for transitions form an initial state |i〉 to a 5d state (these are assumed equal for the 5d3/2 and 5d5/2 states), and h3/2 and h5/2 are the numbers of 5d holes of corresponding character in the valence band. Equations 1 and 2 ignore transitions to s and continuum states and assume spherical symmetry of the atomic potentials. The dynamics of the system affect only the radial integrals R, which are assumed to be approximately constant. The integrated areas δA2 and δA3 of the L2 and L3 difference spectra may now be written as11
δA2 ) ∫δµ2(E) dE ) CNa(Rd2p1/2)2∆h3/2/3 δA3 ) ∫δµ3(E) dE ) CNa(Rd2p3/2)2(∆h3/2 + 6∆h5/2)/15
(3) (4)
where ∆h3/2 and ∆h5/2 are the differences in the number of 5d holes of corresponding character between the supported and bulk metals. Normalization to “unit step height”, which is typically used in XAFS data analysis, will rescale δA2 and δA3 © 1996 American Chemical Society
Support and Temperature Effects in Pt Clusters. 2
J. Phys. Chem., Vol. 100, No. 40, 1996 16319
TABLE 1: XPS Calibration Results (25 eV Pass Energy) line
EB (eV)
fwhm (eV)
line
EB (eV)
fwhm (eV)
Pt(4f7/2) Au(4f7/2) Pt(4d5/2)
71.28 84.00 314.56
1.65 1.35 4.88
Ag(3d5/2) Cu(2p3/2)
368.21 932.66
1.29 1.51
to account for any differences in the radial integrals R, the degeneracy of the initial state, and/or the concentration of absorbing atoms. The ratio ∆h5/2/∆h3/2 is therefore equal to
∆h5/2/∆h3/2 ) (10(δA′3/δA′2) - 1)/6
(5)
The quantities δA′i are the corresponding areas calculated from normalized difference spectra. The extra factor of 2 in eq 5 accounts for the fact that the degeneracy of the initial state has already been included in the numerical coefficients in eqs 3 and 4. Note that eq 5 is valid only if the changes in the integrated areas of the XANES difference spectrum may be attributed entirely to changes in d-band occupancy. We will return to this point in section IIIA. B. XPS. X-ray photoelectron spectra were recorded on a Physical Electronics PHI-550 spectrometer, equipped with a double-pass cylindrical mirror analyzer (CMA) and a cold stage.16 Data were collected with Al KR radiation (hν ) 1486.6 eV, ∆E ) 0.85 eV) in constant retardation mode. The analyzer pass energy was 25 eV, except during measurements of the Pt(4d) lines, where low signal levels necessitated the use of 50 eV pass energy. A scan step size of 0.2 eV/point was used throughout. The nominal energy resolution of the analyzer at 25 and 50 eV is respectively 0.3 and 0.6 eV. The base pressure during measurements was in the range (8-15) × 10-9 Torr during room-temperature measurements and (2-4) × 10-9 Torr during low-temperature (100 K) measurements. The dual-anode X-ray source was operated with a single anode at 15 kV and 250 W. The spectrometer was calibrated with Cu, Ag, Au, and Pt foils. With 25 eV pass energy, the results given in Table 1 could be routinely achieved. The full width at half-maximum (fwhm) of the Pt(4d) lines measured with 50 eV pass energy increased by only 0.2-0.3 eV. A PHI 03-125 flood gun operating at 2 mA was used during all measurements on insulating samples. Binding energies for all insulating samples were referenced to the C(1s) line, which was assigned a binding energy of 285.0 eV. Prior to introduction in the UHV chamber, all samples were treated in situ in an evacuable stainless steel cell described elsewhere.18 The samples were from the same batches used for X-ray absorption measurements and were prepared as described in part 1 of this paper.1 In addition, sample surfaces were cleaned after reduction by mechanical abrasion with an electroplated diamond file under high vacuum prior to insertion in the UHV chamber. All experimental data were smoothed by a nine-point convolution method19,20 with a quadratic polynomial. Inelastic backgrounds were subtracted with the Shirley method.21 Peak positions were determined from the derivatives of the smoothing polynomials,19 and uncertainties in the positions were estimated from the solutions of the equation
P2(x) ) P2(xmax) - 3
(6)
where P2(x) is the smoothing polynomial, is the root-meansquare deviation between the actual data and P2(x), and xmax is the nominal peak position. III. Results and Discussion A. Electronic Properties I: XANES. Normalized XANES difference spectra for all samples listed in Table 1 or ref 1 are shown in Figure 1 for the L2 edges and in Figure 2 for the L3
edges. The sharp features in the vicinity of the edge (E ) 0) are most likely errors due to microstepping problems with the X-11A monochromator. Unfortunately, these problems were not identified until after the data were taken and analyzed, at which point it was not possible to apply corrections. Outside the range within ≈2 eV of the edge the statistical error bars are at most 0.5-1%. The magnitude of the effects at the L3 edge is in agreement with other published data.2,12,13 The change of scale between Figure 1 and Figure 2 is noteworthy for two reasons. First, if the L2 spectrum of a particular data set is compared to the L3 spectrum of the same data set, as in Figure 3, the broad structures in the range 5-15 eV may be immediately identified as “electronic” (DOS) effects, since scattering effects should be of identical amplitude in normalized spectra, as the oscillations above 20 eV are. However, the enhanced unoccupied DOS of the supported metal is not due to charge-transfer effects, as may be seen from the following argument: As seen in Figures 1 and 2, the ratio of the normalized integrated XANES intensities δA′3/δA′2 for the supported metal is at most 1/2, with more realistic estimates placing it somewhere between 1/3 and 1/4. Assuming that the enhanced XANES intensity may be attributed entirely to charge transfer to the zeolite support, and taking 1/2 as an upper limit for the ratio δA′3/δA′2, the upper limit for the ratio ∆h5/2/∆h3/2 may be estimated from eq 5 at 2/3. This implies that charge is removed preferentially from the 5d3/2 level, which is an unexpected result. The spin-orbit splitting of the platinum 5d levels is substantial, between 1.5 and 2.5 eV,7,22-25 which makes preferential removal of 5d3/2 charge unlikely. This viewpoint is supported by the data in Figure 4 and 5, which show respectively XANES difference spectra for several platinum salts, and UPS valenceband spectra for increasing coverage of gold deposited on indium substrates. For all Pt salts, where there is a documented d-character charge transfer to the ligand,10 the ratio δA′3/δA′2 is greater than unity, which places a lower limit on the ratio ∆h5/2/∆h3/2 at 1.5. The peaks in the difference spectra of the Pt salts are at substantially lower energies than the peaks in the corresponding spectra of the supported metal, which also implies that a different mechanism is involved.12 In addition, the results for the Au-In overlayer system show that for coverage as high as six monolayers the Au(5d3/2) level remains virtually unaffected, while the Au(5d5/2) level broadens and shifts to lower binding energies. Both these facts are in agreement with expectations based on simple energetic considerations. We therefore conclude that the enhanced DOS in the supported metal is not due to charge transfer to the support. Certain trends are clearly visible in Figures 1 and 2. The most striking feature is the “doublet” at approximately 6.8 and 12.0 eV. The precise splitting of this doublet it subject to considerable uncertainty, and the assigned position of each of the components can easily be as much as 0.2-0.3 eV in error. In each of the samples where it is present (Pt4H7, Pt4H11, Pt4H20), the intensity of the weak component of the doublet exceeds the noise level in the data. In addition, the consistent appearance of these features in both the L2 and L3 spectra of samples that have similar, but not identical, compositions and pretreatment histories, as well as in data sets taken months apart on samples from different batches, suggests that these are real spectral features of the supported metal. A notable exception is the Pt4H3 sample, which is the only one that was not subjected to NH4Cl exchange.26 It will be seen in section IIIB that a similar grouping of the results also exists for the XPS binding energies of the supported metal. A plausible explanation of the origin of the doublet in Figures 1 and 2 comes from recent work by Hammer and Nørskov,27
16320 J. Phys. Chem., Vol. 100, No. 40, 1996
Boyanov and Morrison
Figure 1. L2 XANES difference spectra for Pt/NaY and Pt/HY samples.
Figure 2. L3 XANES difference spectra for Pt/NaY and Pt/HY samples. Note the change of scale with respect to Figure 1.
who performed ab-initio studies of the mechanism of hydrogen adsorption on the (111) surface of Ni, Cu, Pt, and Au. The main results of these authors are illustrated in Figure 6. First consider the simple two-level interaction problem. When the electronic states of two atoms overlap, a rearrangement of the electron distribution occurs, such that the new electronic states are orthogonal to each other (Pauli repulsion). The overlapping states will also hybridize and will form bonding and antibonding states (Figure 6a). If only the bonding state is occupied, the energy cost of orthogonalization will be offset by the attractive
effect of hybridization, and the new configuration will be stable. However, if some or all of the antibonding states are occupied, little or no hybridization energy is gained, and the orthogonalization energy cost prevails. The splitting of the bonding and antibonding levels increases monotonically with the square of the overlap integral between the two levels.27 The simple picture of two-level interaction is then transferred to the case of hydrogen chemisorption on the metal. The interaction between the metal d bands and the adsorbate levels will induce a filled deep-lying bonding state and an antibonding
Support and Temperature Effects in Pt Clusters. 2
J. Phys. Chem., Vol. 100, No. 40, 1996 16321
Figure 3. Comparison of representative XANES difference spectra: L2 data (s) and L3 data (---).
Figure 5. Valence-band spectra of the Au-In overlayer system for Au coverage up to 13 monolayers. The binding energy is referred to the Fermi level EF. Data are from Boyen et al.23
Figure 6. Schematic illustration of the interaction between electronic states: (a) the simple case of two sharp molecular states and (b) the interaction between an adsorbate and a metal surface. After Hammer et al.27
Figure 4. XANES difference spectra of Pt salts: L2 (s) and L3 spectra (---).
state above the metal d levels (Figure 6b). Fully self-consistent relativistic calculations show that in Au and Cu these antibonding levels lie below the Fermi level, while in Ni and Pt the antibonding levels are a few electronvolts above the Fermi level. Hammer and Nørskov have also calculated reaction energetics for H2 chemisorption on these metals and have shown that, in agreement with experimental results, H2 dissociation on the metal surface is activated on Au and Cu and nonactivated on Ni and Pt. The authors stress that these results are not restricted to H2 chemisorption but should be generally valid for the interaction of transition metal substrates with any atom or molecule with a filled one-electron level below the metal d bands, such as H, C, N, and O.27 Since the size of zeolite-supported Pt clusters is estimated to be between 10 and 25 atoms,1 it is reasonable to expect that the Pt(5d) levels exist as a distinct doublet. This expectation is based on the results presented in Figure 5, where the doublet character of the Au(5d) levels is seen to be preserved for coverage as high as six monolayers.23 Interaction between the Pt(5d) levels and extraframework protons similar to that described in the previous paragraph should therefore lead to a
doublet of unoccupied antibonding states above the Fermi level of the supported metal. This conclusion is in agreement with the observed doublet intensities at the L2 and L3 edges, where the lower-energy component (which presumably originates from the 5d3/2 level) is more intense at the L2 edge, while the higherenergy component dominates at the L3 edge. The presence of a high-energy component at the L2 edge may easily be caused by the lack of full spherical symmetry in the atomic potentials, e.g., due to the crystal field of the zeolite lattice. It is not clear whether this mechanism can account for the anomalous ratio δA′2/δA′3 of the integrated areas of the XANES L2,3 difference spectra. If the doublet in the XANES difference spectra is due to unoccupied antibonding states created by the interaction between Pt(5d) and substrate states, the splitting of the doublet components should be similar to the spin-orbit splitting in Pt (1.52.5 eV). However, the measured splitting is about 5 eV, which is a source of some concern. It should be noted that differences in the overlap between the substrate and the Pt(5d) levels will lead to variations in the splitting between the bonding and antibonding components for each of the substrate-induced states. Due to the greater spatial extent of the Pt(5d5/2) state, its overlap with substrate state(s) will be greater than the overlap of the Pt(5d3/2) state. This difference in overlap will result in different splitting between the bonding and antibonding components of the substrate-induced levels, which may lead to the observed increase in the separation between the antibonding levels. An intriguing feature of the XANES spectrum of supported Pt was first pointed out by Lytle et al.,12 who reported that, unlike bulk Pt, the L3 white line of supported Pt clusters exhibits substantial temperature dependence, as shown in Figure 7. Measurements over an extended temperature range showed that above 600 K δA′3 ≈ 0; i.e., the white lines of the supported
16322 J. Phys. Chem., Vol. 100, No. 40, 1996
Figure 7. Temperature dependence of the XANES difference spectrum of zeolite-supported Pt (Pt4H24 ): 100 K (s) and 300 K (---) data.
and bulk metals are approximately identical.12 It was suggested that the temperature dependence was due to “making and breaking of Pt-O bonds to the support”.12 However, when the detailed temperature dependence of the XANES spectrum is examined (Figure 7), this appears unlikely. Making and breaking of Pt-O bonds would imply changes in the overlap integrals between the orbitals of the metal and the support, which should lead to shifts in the positions and/or separation of the peaks in the doublet, in disagreement with the experimental results. Other mechanisms that may contribute to temperaturedependent changes in the width of the XANES white line and are consistent with the invariance of the doublet position include the following: (i) Inhomogeneous Adsorption-Site Broadening. This mechanism was first suggested by Mason.28 If a distribution of nonequivalent adsorption sites for small clusters is present in the support, each of which produces a unique chemical shift in the spectrum, the averaging over all sites will result in a broadening of the measured spectrum. As the temperature is increased, the cluster will be able to “hop” to energetically favorable sites, which should result in decreased white line width. An additional side effect of such preferential population of low-energy adsorption sites would be a decrease in the line widths of the doublet in the XANES difference spectrum, which indeed seems to be the case (Figure 7). However, the magnitude of this effect is comparable to the noise level in the data, so no firm conclusions can be made. Valence-band UPS and XPS measurements may be helpful to assess the viability of this hypothesis. (ii) Surface-Atom Chemical Shifts. Because of their different environment, surface atoms are known to exhibit distinct XPS chemical shifts.29 It has been suggested that such chemical shifts may be partially responsible for the observed broadening of XPS core-level lines of supported metals.30 However, the clusters studied here consist almost exclusively of “surface” atoms. If inhomogeneous interactions with the support are not important,30 core-level XPS lines for such clusters should exhibit little or no broadening. This conclusion is not supported by the experimental results, as will be seen in section IIIB. It is also not clear how this broadening mechanism can account for the temperature dependence of XANES spectra. (iii) Electron-Scattering and Thermal-Expansion Effects. Electron scattering may lead to broadening of the white line in two ways. A valence electron moving at the Fermi velocity VF
Boyanov and Morrison in a cluster of size L will scatter from the boundary of the cluster in time τ ) L/VF. This means that plasmon oscillations in the cluster lose coherence in time τ, and therefore the spectral width of the plasmon loss function for the cluster will be broadened in comparison to that of bulk metal by δE = p/τ, which will lead to size-dependent broadening of the absorption and emission lines of the supported metal.30 Such size-dependent broadening of the XPS lines of Pt and Pd supported on amorphous carbon has indeed been established.30 Anomalously high thermal expansion of the cluster (e.g., due to the reduced coordination) could then lead to a temperature-dependent broadening. To the best of our knowledge, no comparable absorption measurements have been published. Scattering of the ejected photoelectron from the atomic potential within the cluster may also lead to structure in the XANES region.31 If any of the latter three mechanisms have measurable contributions to the integrated XANES intensity of the supported metal, serious questions are raised about the limits of applicability of XANES-based methods for quantitative estimation of d-band occupancy.11 It is therefore important to determine whether effects other than DOS modifications can measurably affect the intensity of XANES difference spectra of supported metals. B. Electronic Properties II: XPS. A series of core-level XPS lines of the zeolite and the supported metal are accessible with Al KR radiation. In order to separate the effects of the supported metal on the zeolite lattice (and Vice Versa) from the effects of acidity, Pt-loaded samples and Pt-free zeolite “blanks” of identical acidity and thermal history were measured, at both room temperature and 100 K. The room-temperature line positions and widths for all primary XPS lines of framework and Na atoms are given in Tables 2 and 3. Numerical values for the Auger parameters32 are given in Table 4. The reference binding energies for Na-exchanged zeolite Y of Barr et al.33 have been shifted by 0.6 eV to account for the change in energy reference.34 The differences in line widths may be attributed to the unmonochromatized Al KR source used in this work. Similar binding energies and line widths for Si, O, and Al in zeolite Y are reported elsewhere.35,36 The small increase in the binding energies of the Pt-free zeolites with framework acidity is in agreement with previous reports.36 However, since the increase is comparable to the experimental error bars (0.1-0.2 eV), no particular significance can be attached to that result. The binding energies for Pt-loaded zeolites are approximately independent of the framework acidity, with the scatter in the results similar to the reproducibility in the line positions during repeated measurements of independently prepared samples of the same batch, which is better than (0.10-0.15 eV. The Auger parameters are also constant within the experimental uncertainty. The most intense and sharp XPS line of Pt, the Pt(4f) doublet, completely overlaps the Al(2p) line (an unresolved doublet), as shown in Figure 8. Previous studies of zeolite-supported Pt clusters have attempted to track Pt(4f) line positions from shifts in the peak positions of the combined Pt-Al envelope.35 However, this approach does not allow changes in the position of both the j ) 7/2 and j ) 5/2 components of the line to monitored, so a different procedure was adopted in this work. First, a scan in the range 55-130 eV (BE) is taken, both for Pt-loaded and Pt-free zeolites. Then the invariant parts of these spectra (the Al(2s) and Auger lines and the Si(2p) line) are rescaled to the same intensity and are aligned horizontally by a rigid shift. The scaling and shift factors are then used to rescale and align the energy range spanned by the Pt-Al envelope in both scans (≈65-80 eV). The data for the backgroundsubtracted Pt-free zeolite is then subtracted from that of the
Support and Temperature Effects in Pt Clusters. 2
J. Phys. Chem., Vol. 100, No. 40, 1996 16323
TABLE 2: XPS Binding Energies for Framework and Na Atoms in Pt-Free Zeolite Ya H3
H7
H11
referenceb
H20
line
EB
fwhm
EB
fwhm
EB
fwhm
EB
fwhm
EB
fwhm
Na(2s) Al(2p) Al(A) Si(2p) Al(2s)c Na(A) O(A) O(A) Na(1s)
64.6(1) 74.6(1) 93.1(2) 103.0(1) 119.5(1) 498.8(1) 532.2(1) 979.3(2) 1072.8(1)
2.3 2.1
64.6(1) 74.8(1) 93.3(2) 103.2(1) 119.6(1) 499.0(1) 532.4(1) 979.6(2) 1073.0(1)
2.4 2.2
64.8(1) 74.8(1) 93.3(2) 103.2(1) 119.6(1) 499.0(1) 532.4(1) 979.6(2) 1073.0(1)
2.4 2.2
64.7(1) 75.0(1) 93.3(2) 103.2(1) 119.7(1) 498.8(1) 532.4(1) 979.5(2) 1073.0(1)
2.2 2.2
64.40 74.60
1.65 1.55
2.2
102.95
1.68
2.5
532.15
2.05
a
2.2 2.5 2.5
2.3 2.6 2.5
2.3 2.6 2.4
All energies are in eV and are referenced to C(1s) ) 285.0 eV. From Barr et al. b
33
2.3
Values are ( 0.15 eV. Partially overlaps Si(A). c
TABLE 3: XPS Binding Energies for Framework and Na Atoms in Pt-Loaded Zeolite Ya Pt4H3
Pt4H7
Pt4H11
referenceb
Pt4H20
line
EB
fwhm
EB
fwhm
EB
fwhm
EB
fwhm
EB
fwhm
Na(2s) Al(A) Si(2p) Al(2s)c Na(A) O(1s) O(A) Na(1s)
64.6(1) 93.2(2) 103.2(1) 119.6(1) 498.8(1) 532.4(1) 979.4(2) 1072.8(1)
2.3
64.6(1) 93.2(2) 103.2(1) 119.7(1) 498.8(1) 532.4(1) 979.4(2) 1073.0(1)
2.3
64.6(2) 93.1(2) 103.2(1) 119.7(1) 498.8(1) 532.4(1) 979.4(2) 1072.9(1)
2.5
64.6(1) 93.2(2) 103.2(1) 119.6(1) 498.7(1) 532.4(1) 979.4(2) 1072.9(1)
2.4
64.40
1.65
2.2
102.95
1.68
2.5
532.15
2.05
a
2.2 2.6 2.3
2.3 2.6 2.4
2.2 2.5 2.3
All energies are in eV and are referenced to C(1s) ) 285.0 eV. From Barr et al. b
33
2.3
Values are (0.15 eV. Partially overlaps Si(A). c
TABLE 4: Auger Parameters r′ for Framework and Na Atoms in Zeolite Ya parameter
H3
H7
H11
H20
Pt4H3
Pt4H7
Pt4H11
Pt4H20
R′O (1s+A) R′A (2s+A) R′Na (1s+A)
1039.5 1513.0 2060.6
1039.4 1512.9 2060.7
1039.4 1512.9 2060.6
1039.5 1513.0 2060.8
1039.6 1513.0 2060.6
1039.6 1513.1 2060.8
1039.6 1513.2 2060.7
1039.6 1513.0 2060.8
a
All energies in eV.
Figure 8. XPS spectrum of zeolite Y before (top) and after (bottom) Pt exchange. Both samples are of identical framework acidity and thermal history.
background-subtracted Pt-loaded zeolite. The results are shown in Figure 9. The subtraction procedure is justified by the fact that the Si and Al content of all samples is identical, and the separation between the various lines in the the Pt-free and Ptloaded samples is also identical to within 0.1 eV. In addition to the 4f doublet, the Pt(4d) lines were also measured for the supported metal and bulk Pt. However, the Pt(4d) lines are substantially wider and less intense than the 4f lines (Table 1), which results in greater uncertainty in the estimated binding energies. The Pt 4d lines of the supported
Figure 9. Platinum 4f doublet for zeolite-supported Pt clusters (s) and bulk Pt (---). The curve at the bottom indicates the noise level in the data.
metal are shown in Figure 10. Numerical results for the binding energies and line widths are given in Table 5. The cluster 4d and 4f lines are shifted with respect to the corresponding bulk lines to higher binding energy by 0.6-0.9 eV, which is comparable to previously published results.35,37 There is very little or no dependence of the binding energy on the acidity of the support. The 0.2-0.3 eV positive BE shift between Pt4H3 and the other zeolite samples is reproducible to within 0.1 eV over repeated measurements of independently prepared specimens. However, since the effect is comparable the experimental uncertainty, it is once again not possible to attach any significance to this result. It is nevertheless important to note
16324 J. Phys. Chem., Vol. 100, No. 40, 1996
Figure 10. Platinum 4d doublet for zeolite-supported Pt clusters (s) and bulk Pt (---). The curve at the bottom indicates the noise level in the data.
that, as with the XANES results, the effect is comparable for both the 4d and 4f lines, and the samples appear to divide in two groups, according to whether [Pt(NH3)4]Cl2 exchange has been preceded by NH4Cl exchange. The Auger lines of Pt are not accessible with Al KR radiation,38 so it is not possible to determine whether this is an initial- or final-state effect. In an attempt to determine the origin of the temperature dependence of the XANES spectrum, temperature-dependent XPS valence-band measurements were also performed on all samples. The XPS valence-band spectrum of the zeolite contains contributions primarily from framework atoms, but at 5.6-6.0 wt % loading Pt atoms give a measurable signal. Room-temperature spectra for all samples are shown in Figure 11. As the signal levels are very low (each of these spectra was collected over a period of 15-20 h), the detailed shapes of the spectra are subject to considerable uncertainty,39 and no significance should be attached to small variations in the overall spectral distribution across the sample series. The contribution of Pt to these spectra was extracted with a subtraction procedure identical to that used for the Pt(4f) doublet. The broad features in the range 18-30 eV (not shown in the figures) were used to scale and align the spectra. These features have been attributed to framework oxygen atoms39 and should therefore be approximately invariant in all spectra. The difference spectra, shown in Figure 12, bear close resemblance to the valence band of bulk Pt. Despite the significant noise level in these data (estimated from the sum of the noise levels in the subtracted spectra), two characteristic features of the valence band of supported Pt consistently appear above the noise level across the sample series and are reproducible upon repeated measurements of independently prepared specimenssthe valence band narrows, and its leading edge shifts to higher binding energies in comparison to bulk Pt. The shift in the leading edge of the valence band of the supported metal mirrors the measured core level shiftssapproximately 0.5-0.6 eV for Pt4H3 and 0.7-0.9 eV for the other samples. The core-level BE shifts of the supported metal therefore appear to originate primarily in a shift in the reference level, as first suggested by Citrin et al.,29,40 not in charge transfer between the cluster and the support. Repeated measurements of core and valence-band spectra at 100 K failed to detect temperature dependence exceeding the noise level in any of the data. However, it should be noted that the expected magnitude of any effect is only a few percent (Figure 7), which is comparable to the noise level. A conclusive measurement would require much better counting statistics,
Boyanov and Morrison possibly even a synchrotron source. As was the case with XANES, the XPS valence-band spectrum of bulk Pt exhibits no temperature dependence.41 Other temperature-dependent studies of the zeolite valence band have been published,42 but just like the measurements in this work, the statistical quality of the data is not sufficient to make any statements about the temperature dependence of the valence-band spectrum of the zeolite precursor. C. Electronic Properties III: AXAFS. The AXAFS (atomic XAFS) technique is still in its infancy, and its utility as a probe of the electronic structure has not yet been proven. AXAFS has been used to identify many-electron excitations in X-ray absorption spectra,44 and it has been suggested that it may be a useful probe of the electron distribution “in the periphery of the absorbing atom”.43 The effect is like an internal Ramsauer-Townsend resonance, where the incident photoelectron is a spherical wave emanating from the center of the atom, rather than a wave scattered by the atom.43 As the photoelectron approaches the potential barrier at the edge of the atom, the reflection coefficient oscillates with energy, which leads to a modulation in the atomic background. As discussed by Rehr and co-workers,43 the inclusion of AXAFS effects in the background is particularly important when quantification of asymmetry in the atomic radial distribution is to be pursued. Since both of these effects (asymmetric disorder and charge redistribution) are of interest in the present study, it is important to isolate any AXAFS that might be present. The present study also provides the opportunity to verify the atomic origin of these effects and to investigate their dependence on small changes in the electronic structure. The AXAFS backgrounds of the supported metals extracted as described in ref 1 are shown in Figure 13. To facilitate comparison, a linear background has been subtracted from the range 250-350 eV (not shown in the figure). The backgrounds have typical AXAFS structuresa sharp initial oscillation (40 eV) is followed by one or more broad oscillations (170 eV). When not accounted for, this structure results in a spurious peak in the EXAFS spectrum at approximately one-half the nearneighbor distance, as predicted by Rehr et al.43 The reproducible steplike features at approximately 80 and 110 eV most likely correspond to double-electron ionization, 2p3/24f and 2p3/25s, respectively. The expected positions of these features may be estimated with the Z + 1 approximation,44 where the atomic levels of the Z + 1 element (in this case Au) are used to approximate the ionization energies of the Z element with a hole in a core level. Within this approximation, the 2p3/24f double-ionization threshold should be in the range 84-90 eV above the L3 absorption edge (an unresolved doublet), while the 2p3/25s threshold should be at approximately 110 eV above the L3 edge.45 No systematic trends are visible across the sample series. If AXAFS is truly sensitive to the charge distribution at the periphery of the atom, this would imply the lack of any systematic charge transfer as a function of zeolite acidity. However, at this time it is not clear how sensitive AXAFS is to small changes in the electronic structure. Furthermore, the noise level in the data precludes the reliable identification of effects at the few percent level. It is important to note the lack of any temperature dependence in the AXAFS background (Figure 13). This confirms the atomic origin of the effect, since the EXAFS spectrum exhibits unambiguous temperature effects over this energy range. It was not possible to extract AXAFS backgrounds of comparable signal-to-noise ratio from Pt foil data. This is due to the presence of numerous multiple-scattering paths in the data range R < 5 Å, which cannot be modeled reliably during
Support and Temperature Effects in Pt Clusters. 2
J. Phys. Chem., Vol. 100, No. 40, 1996 16325
TABLE 5: XPS Binding Energies for Pt(4f) and Pt(4d) Levelsa Pt4H3 line
EB
Pt(4f7/2) Pt(4f5/2) Pt(4d5/2) Pt(4d3/2)
71.9(2) 75.3(2) 315.0(3) 331.9(3)
a
Pt4H7 fwhm
EB
5.0 6.0
72.1(2) 75.4(2) 315.3(3) 332.3(3)
Pt4H11 fwhm
EB
5.0 6.3
72.1(2) 75.5(2) 315.3(3) 332.3(3)
Pt4H20 fwhm
EB
5.0 5.7
72.2(2) 75.4(2) 315.4(3) 332.2(3)
Pt foil fwhm
EB
fwhm
5.1 6.1
71.3(1) 74.7(1) 314.6(2) 331.4(2)
1.9 2.0 4.8 5.1
All energies are in eV and are referenced to C(1s) ) 285.0 eV.
Figure 11. XPS valence-band spectra for Pt-loaded (s) and Pt-free (---) zeolite Y at room temperature. The curves at the bottom indicate the noise level in the data.
Figure 12. XPS valence-band difference spectra for zeolite-supported Pt (s). The valence band of bulk Pt (---) is shown for comparison. The curves at the bottom indicate the noise level in the data.
background subtraction because of severe parameter correlation and restrictions on the maximum number of independent parameters. The omission of these paths results in residual oscillations in the AXAFS background that obscure weak features. This appears to be one of the major sources of noise
Figure 13. AXAFS backgrounds for zeolite-supported Pt clusters: 100 (s) and 300 K (---) data.
in the proposed method for extraction of AXAFS backgrounds for crystalline materials. IV. Summary and Conclusions The influence of framework acidity on the spatial and electronic structure of platinum clusters supported on zeolite Y
16326 J. Phys. Chem., Vol. 100, No. 40, 1996 has been studied with a combination X-ray absorption and photoelectron spectroscopy and ab-initio simulations. The clusters contain 10-25 Pt atoms on average. The Pt-Pt bond length for “bare” clusters is 2.70 ( 0.01 Å. While certainly present in the supported metal, static disorder is shown to be symmetric on average. The 0.07 Å contraction in the bond length with respect to bulk Pt is therefore deemed to be intrinsic to the cluster. X-ray absorption near-edge (XANES) spectra for a series of samples with increasing framework acidity exhibit systematic trends at the 5-10% level. Within the framework of the chargetransfer model, the observed effects are shown to imply preferential removal of 5d3/2-type electron density over 5d5/2type density. Due to the large spin-orbit splitting of the Pt 5d levels (1.5-2.5 eV), such charge transfer is inconsistent with expectations based on simple energetic considerations. It is therefore concluded that XANES modifications are not caused by charge transfer between the cluster and the support. An interpretation in terms of interaction between the cluster and the support and the subsequent creation of unoccupied antibonding levels above the Fermi level of the supported metal is proposed. These results suggest that presently used XANESbased methods for the quantitative estimation of d-electron count may be inapplicable to supported transition metals. X-ray photoelectron spectroscopy (XPS) results show a 0.20.3 eV systematic increase in the core-level binding energies of framework atoms (Si, O, Al) with acidity, while the corresponding Auger parameters remain constant. The 4d and 4f binding energies of supported Pt, when referenced to C(1s) EB ) 285.0 eV, are shifted by 0.6-0.9 eV relative to the corresponding lines of bulk Pt. The 4d and 4f lines of Pt supported on proton-loaded zeolite Y are found to be 0.2-0.3 eV higher than the corresponding lines of Pt supported on asreceived zeolite Y. XPS valence-band spectra of the supported metal were measured and found to be more narrow than the corresponding spectrum of bulk Pt. The shifts in the leading edge of the valence band of supported Pt are identical to shifts in the core lines. It is therefore concluded that the observed shifts are due to a change in the reference level for small clusters. The estimated experimental uncertainty in the binding energy shifts is 0.1-0.2 eV and is limited both by the signal-to-noise ratio of the data and the reproducibility of the results over repeated measurements of independently prepared identical samples. Atomic X-ray absorption fine structure (AXAFS) was extracted from the data and analyzed. The atomic origin of the effect was confirmed with temperature-dependent measurements, and 2p3/24f and 2p3/25s double-electron ionization thresholds were identified. The AXAFS backgrounds exhibit no dependence of zeolite acidity, giving further evidence for the lack of systematic charge transfer between the metal and the support as a function of zeolite acidity. In an attempt to determine the origin of previously reported temperature effects in the XANES spectrum of supported Pt, temperature-dependent XANES and XPS measurements were performed. The XANES results suggest that there is no evolution of the electronic structure of the supported metal with temperature. The XPS measurements were inconclusive due to poor counting statistics. Further studies are necessary to determine the origin of the temperature dependence of the XANES spectrum and the broadening of core-level XPS lines. Acknowledgment. We thank Drs. G. Bunker (IIT) and J. Faber (UIC) for their active interest in this work and numerous stimulating discussions. This work was made possible in part by funds provided by Amoco Corp. The authors acknowledge use of the Argonne High-Performance Computing Research Facility (HPCRF) and beamline X-11 at the National Synchro-
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